Jet engines are used all over the world for different types of airplanes. There are different types of jet engines, and a turbofan is one of them. The fanjet or turbofan engine is a famous type of engine from the category of jet engine. A turbofan engine is most commonly utilized in aircraft propulsions. The turbofan has an additional fan which helps to accelerate a lot of mass without combusting extra fuel. This article explains the turbofan engine working, types, components, and applications.
What is a Turbofan?
A turbofan engine is a modified type of jet engine that uses a combination of bypass air and jet core efflux to create thrust. Bypass air is blown through a ducted fan. The jet core drives this ducted fan. The turbofan engine is also known as a bypass or fanjet engine.
The term “turbofan” stands for “turbine” and “fan,” the “turbine” part represents a gas turbine that obtains mechanical power through combustion, and the part “fan” represents a ducted fan that utilizes the gas turbine’s mechanical power to accelerate the air backward.
The fan of the turbofan draws excess power from the exhaust gases through the turbine. This process slows down the exhaust speed slightly, but bypass air increases the mass significantly.
In the case of the turbojet, all of the drawn air will flow through the turbine and combustion chamber. In contrast, in a turbofan engine, only some part of the air drawn into the engine bypasses through the turbine and combustion chamber. Therefore, a turbofan engine works as a turbojet that uses a ducted fan.
These engines generate thrust through a propulsion system (fan) and move the aircraft by air. The latest airplanes have turbofan engines because these engines have good fuel efficiency and generate high thrust.
The turbofan engine efficiency can measure as the ratio of the fuel burned to generate a required amount of thrust. It’s usually expressed as a pound of fuel per pound of thrust.
Most commercial aircraft engines nowadays have high-bypass engines, but the latest military fighter jets have low-bypass turbofan engines. Afterburners can’t utilize in high-bypass turbofans but can be utilized in low-bypass engines.
In this type of engine, the core engine surrounds by a front fan and an additional turbine at the rear. Turbines and fans have multiple blades like a central turbine and a central compressor linked through an extra shaft. Because of some mechanical reasons, the fan shaft passes by a central shaft. This arrangement of engine knows as a 2-spool engine. One “spool” is for the core, and the other “spool” is for the fan.
How Does a Turbofan Work?
The working of the turbofan engine is very similar to the turboprop engine. A turbofan engine works in the following way:
- Firstly, the air is sucked inside the engine via a fan, and air divides into two various paths.
- Some part of the air enters the engine core where combustion takes place while the remaining part of the air (bypass air) moves outside of the engine core by a duct.
- After suction, the air enters into a low-pressure compressor which increases the pressure of the air according to the requirements and sends it into a high-pressure compressor.
- When the low-compressed air comes into the high-pressure compressor, it further compresses the air up to very high pressure and also high increases its temperature.
- The high-pressure compressor makes the air temperature so high that when it touches the fuel in the combustion chamber, the combustion process starts itself.
- After the combustion of the air-fuel mixture, the combusted gas enters into the low-pressure and high-pressure turbine.
- After entering the turbine, the hot gases expand into the turbine and strike the turbine blades. The turbine blades extract enough power from the combusted mixture to move the low-pressure compressor and fan. The remaining power of the combusted mixture is sent toward the exhaust nozzle.
- When the exhaust gases enter the nozzle, the nozzle converts their pressure energy into speed and converts them into very high-speed gases.
- When high-speed gases are discharged from the nozzle into the atmosphere, they generate thrust, which moves the airplane forward.
- The speed of the air flowing by the fan is slightly faster than the speed of the air flowing free. This flow of air is known as the bypass or fan airflow.
Construction of Turbofan Engine
The Turbofan engine was designed to avoid the unwanted properties of the subsonic flight efficiency of turbojet engines.
An obvious way to improve turbojet engine efficiency is to raise the burner temperature to improve Carnot efficiency and fit the nozzle and larger compressor. This process increases the thrust, but the exhaust gases exit the engine with a high velocity which consumes additional engine power.
Due to the above-given reasons, the jet engine consumes a high amount of fuel. These engines have slow speed and low efficiency. Therefore, scientists designed the turbofan to avoid extra fuel consumption and improve fuel economy.
Types of Turbofan Engines
The turbofan has the following three major types:
- Low-bypass engine
- Medium or afterburning engine
- High-bypass engine
1) Low-bypass Turbofan
A type of engine that uses jet thrust more than fan thrust is called a low bypass turbofan. The low-bypass-ratio turbofan engine contains a multi-stage fan that generates comparatively high-pressure ratios, resulting in higher exhaust gas velocities (cold or mixed). The airflow from the core must be large enough to deliver adequate core power to run the fan.
In this engine, a higher bypass ratio/lower core flow cycle can achieve by increasing the inlet temperature of the HP (high-pressure) turbine rotor.
These types of engines have high fuel efficiency compared to basic turbojet engines. The latest fighter aircraft mostly have low bypass ratio turbofan engines with afterburners. After that, they can navigate effectively but still have plenty of thrust on dogfighting.
The fighter aircraft may move much faster than the sound speed, but for extreme efficiency, the air entering the engine must travel at a speed slower than the sound speed.
2) Afterburning Turbofan
The main purpose of afterburners is to raise the thrust normally used for supersonic flight, combat, and takeoff.
After the combustion, additional fuel is injected into the combustion chamber in the injection pipe downstream of the turbine to “rewarm” the exhaust gases. It highly boosts the thrust with weight loss rather than using a larger engine. However, this engine consumes a high amount of fuel due to that it uses for a short time.
3) High-bypass Turbofan
An engine whose fan thrust is much larger than the jet thrust is called a high bypass turbofan. To improve fuel consumption and reduce noise, almost all current passenger aircraft and military aircraft (such as the C-17) have high-bypass turbofan engines.
These types of turbofan engines are developed from the low-bypass ratio turbofan engines used in the aircraft of the 1960s.
A low definite thrust is attained in this engine by changing the multi-stage fan with a single-stage fan. In contrast to some military engines, the latest civil turbofans do not have a fixed intake guide vane on the fan rotor’s front. The fan has a fixed scale to attain the required net thrust.
The engine’s core (or gas generator) must produce adequate energy to run the fan at the desired pressure ratio and flow rate. Modifications in turbine material/cooling technology increase the inlet temperature of the higher-pressure (HP) turbine rotor, which enables smaller (lighter) cores and (potentially) improves core thermal productivity.
A decrease in core mass flow manages to increase the load on the low-pressure turbine, so this unit will need extra stages to maintain the low-pressure turbine’s efficiency and decrease the average stage load. The reduction in core flow also raises the bypass ratio.
Components of Turbofan Engine
The turbofan engine has many components, but the most common components are given below:
- Fan
- Compressor
- Turbine
- Shaft
- Combustion chamber
- Nozzle
1) Fan
A fan is a most important component of turbofans. This part helps the engine to generate thrust.
A fan is the first component of turbofans. You can see this in the below-given diagram, and you can also find this part at the front of the aircraft.
The fan blades are most usually construed of titanium alloy. These can draw large amounts of air inside the engine.
Air flows through the engine’s two parts. Some portion of the air directs to the core of the engine, where ignition occurs. The remaining air (referred to as “bypass air”) flows through a ducted fan on the external side of the engine’s core. This bypass air creates additional thrust, cools the engine, and calms it down by removing the engine exhaust gases. In the latest fanjet, maximum engine thrust is created by bypass air.
2) Compressor
The compressor plays an important role in the working of the turbofan engine. The main objective of the compressor is to increase the pressure and temperature of the air.
A turbofan engine uses two compressors (low-pressure compressor and high-pressure compressor) for air compression. These are centrifugal compressors. This centrifugal compressor has a series of rotating blades shaped like an airfoil to compress and accelerate the air. The major parts of the centrifugal compressor are the impeller, inlet port, diffuser, and discharge port.
As air passes across the compressor, the impeller blades of the compressor become smaller during this process. They add energy into the air and compress it. Due to this process, the pressure and temperature inside the combustion chamber increase.
The compressor has a series of fixed or stator blades. These stator blades receive high-velocity air by the impeller and convert this velocity into air pressure. The stators also prepare air for entry into the next fixed blades. In simple words, the stator blades “straighten” the airflow.
Read More: Different Types of Compressors
3) Combustion Chamber
A combustion chamber is a place inside the engine where the combustion takes place. When the air leaves the compressor and arrives inside the combustion chamber, it mixes with the fuel and ignites.
It sounds simple, but in reality, it is a highly complex procedure. This is because the burner must continue providing stable burning of the air-fuel mixture while the air flows by the burner at a very high speed.
The engine housing has all the components of the burner in which the diffuser is the component that works first.
The diffuser slows the air in the compressor so that it can burn easily. Domes and vortices increase air turbulence and make it easier to mix with the fuel. As you can imagine, a fuel injector injects fuel to mix with the air, and an ignition process can occur.
From there, the actual combustion takes place in the lining. The liner has multiple air inlets through which air can appear from different points in the combustion area.
The last important component is the lighter. This is very similar to spark plugs for automobiles and piston-engine airplanes. After the lighter ignites the fire source, it is self-sufficient and turns off the lighter (although it is generally used as a backup in bad weather or icy conditions).
4) Turbine
As the combusted air leaves the combustion chamber, it enters the turbine. A turbine is a series of aerodynamic blades that are very similar to compressor blades. As high-velocity hot air runs through the blades of the turbine, they absorb additional energy from the air, causing the turbine to rotate completely and rotates the engine shaft connected with it.
The turbine shaft is also connected to the fan and compressor. As the turbine spins, the compressor and fans at the front of the turbofan engine continue to suck in more air and quickly mix with the fuel and burn.
The turbines require an additional fan (as shown in the above diagram), which results in more giant turbines and larger temperature and pressure drops, which results in smaller nozzles. This signifies that the core escape velocity will decrease.
Read More: Different Types of Turbines
5) Nozzle
The nozzle is the last part of the fanjet engine. The nozzle is a most important component of a jet engine as well as a turbofan engine. This component of the engine is vital because it generates thrust by expelling exhaust gases into the atmosphere at high speed, which helps the airplane move forward.
This process works according to Newton’s third law. According to this law, each action has an equal but opposite direction reaction. Therefore, when the nozzle expels the air at high speed, the air also exerts an equal but opposite directional force and moves the aircraft forward.
What is a Bypass ratio?
The bypass ratio (BPR) is a comparison of the mass flow rate of the air that bypasses through the core with the mass flow of air drawn in the engine core. For example, if the bypass ratio is 12:2, it means that 12 kg of air will bypass the engine core and 2 kg of air will flow through the core.
The turbofan engine is divided into different types on the base of bypass ratio, which, together with the fan pressure ratio, the turbine inlet temperature, and the total pressure ratio, are vital design factors. Turboprop engines and un-ducted fan units also use BPR because of their high propulsion effectiveness providing them with the overall efficiencies features of excessive bypass turbofans.
The bypass ratio can also be used to install a lift fan in which the fan airflow is away from the motor and does not flow through the motor core. A high BPR decreases the burning of the fuel for the same thrust.
An aircraft is well suited to supersonic speed when all of the gas output from the gas turbine is transformed into K.E. in the propelling nozzle. An airplane is best at zero speed when gas out is transformed into a single large mass and low kinetic energy.
Turbofan VS Turbojet
The main difference between Turbofan and Turbojet is given below:
| Turbofan | Turbojet |
|---|---|
| A turbofan engine has low noise than a turbojet. | A turbojet has high noise. |
| It uses a fan to generate thrust. | It uses its exhaust to generate thrust. |
| It is a modified version of the turbojet. | A turbojet engine is an old version of a jet engine. |
| A turbofan has high efficiency. | A turbojet has lower efficiency. |
| It has high fuel efficiency. | It has lower fuel efficiency. |
| It uses an additional fan to produce thrust. | There is no need for an additional fan. |
| It generates more thrust than a turbojet. | It generates less thrust. |
| These engines are employed in bombers and airliners. | These engines are employed in fighter aircraft. |
Difference between a Turbofan Engine and a Turboprop Engine
| Turbofan Engine | Turboprop Engine |
|---|---|
| A turbofan has a fan located in the front. | A turboprop has a propeller attached to the front. |
| It uses a fan to produce thrust. | It uses a propeller to generate thrust. |
| This engine has high efficiency of fuel. | It has low fuel efficiency. |
| A turbofan engine is less efficient than a turboprop. | A turboprop is more efficient. |
| The aircraft having turbofans run at high speed. | The aircraft having turboprop run at a slow speed. |
| It has a lower bypass ratio. | It has a higher bypass ratio. |
| These engines are used in high-speed subsonic aircraft. | These engines are used in lower or small-speed subsonic aircraft. |
Applications of Turbofan Engine
- Turbofan engines are also employed as ground power units (GPUs) or auxiliary power units (APUs) to produce electricity for different applications.
- They are employed in military aircraft for multiple purposes, such as combat missions, reconnaissance, and transport.
- Turbofan engines are most commonly used in commercial airliners.
- In business jets, they are utilized to supply efficient propulsion for high-speed, long-range flights.
- Turbofan engines are also used to power some types of Unmanned Aerial Vehicles (also known as drones).
- Small turbofans are used to power regional jets, which are usually utilized for short to medium-range flights with the minimum number of passengers.
- They are used in the development of supersonic and hypersonic aircrafts, such as the now-retired Concorde, and the ongoing research into next-generation high-speed passenger aircraft.
FAQ Section
What are the advantages of a turbofan engine?
- These engines have very high efficiency at subsonic speed.
- The aircraft having turbofans run at high speed.
- They have high efficiency of fuel.
Where are turbofan engines used?
A turbofan engine uses on wide-body airliners such as the Airbus A300 and the Boeing 747.
What type of aircraft are turbofan engines most widely used on?
Approximately all passenger airplanes use turbofan engines. This engine was designed to rotate a large fan or series of fans in the engine front to generate approximately 80% of the engine’s thrust. It has very low noise and has excellent fuel economy in this speed range.
Can turbofans go supersonic?
Turbofan engines can withstand supersonic speed because the inlet generates constant flow states regardless of flight speed. In subsonic flow conditions, the fan blades and propellers have the highest efficiency.
What drives the fan in a turbofan engine?
In the turbofan, the low-pressure turbine drives the low-pressure compressor, which further drives the fan.
Why are turbofans quieter than turbojets?
A turbojet has high noise than a turbofan for the same level of thrust. Turbofan’s thrust is generated by rotating a fan in a turbine engine that accelerates a large amount of air to a slower speed than a turbojet engine. Therefore, for the same thrust, the discharge of the fanjet has lower energy when it leaves the engine. Therefore, it generates lower noise than a turbojet.
Which is better, turboprop or turbofan?
The turboprop engine has high efficiency than the turbofan engine. However, an airplane having a turbofan has high speed than a turboprop engine.
What are the types of Turbofan Engines?
- Low-bypass engine
- Medium or afterburning engine
- High-bypass engine